Experimental and Numerical Evaluation of the Cavitation Performances of Self-Excited Oscillating Jets

Self-excited oscillating jets(SOJ)are used in several practical applications.Their performances are significantly affected by structural parameters and the target distance.In this study,a geometric model of the SOJ nozzle accounting for multiple structural parameters is introduced,then the related cavitation performances and the optimal target distance are investigated using a Large-Eddy Simulation(LES)approach.Results are also provided about an experiment,which was conducted to validate the simulation results.By analyzing the evolution of the vapor volume fraction at the nozzle outlet,a discussion is presented about the effect of the aforementioned structural parameters on the cavitation performances and the target distance.It is shown that the distribution of cavitation clouds at the outlet of the SOJ nozzle displays a non-monotonic trend(first increasing,then decreasing).Under working conditions with an inlet pressure of 4 MPa,a SOJ nozzle outlet/inlet diameter ratio(D_(1)/D_(2))of 1.2,and a chamber diameter ratio(D/L)close to 1.8,the nozzle outlet cavitation performance attains a maximum.The optimal structural parameters correspond to the optimal target distance,which is near 50 mm.The experiments have revealed that the SOJ nozzle with the above parameters displays a good cavitation erosion effect at the target distance of 50 mm,in satisfactory agreement with the numerical simulation results.

THEORY AND EXPERIMENTAL STUDY OF THE SELF-EXCITED OSCILLATION PULSED JET NOZZLE

Comparing with usual continuous jet nozzle, the self-excited oscillationpulsed jet nozzle SEOPJN) can make jet generate a higher peak of pressure and larger scouringvolume. And it can make jet increase the effective standoff distance, too. The basic theories of theSEOPJN are introduced. Some experimental results are shown. According to the results, using tricornbits assembled the SEOPJN to drill oil well, the ROP increases by 8 percent approx 77 percent, andthe rates of the footage for tricorn bit increases by 6.7 percent approx 44.0 percent.

The Theory and Experimental Study of the Self-Excited Oscillation Pulsed Jet Nozzle (Pipeline Pulsed Flow Generator)

In this paper, the basic theories of the Self-excited Oscillation Pulsed Jet Nozzle (SEOPJN) invented by the authors are introduced. Then, some experimental results are shown. According to the results, using tricorn bits assembled the SEOPJN to drill oil well, the rate of penetration (ROP) increases by 8% - 77%, and the rate of the footage for tricorn bit increases by 6.7% - 44%. Although the test was conducted in the water, good result was got in nature gas transportation. The volume of gas transportation could be increased by the Self-excited Oscillation Pulsed generator while the gas pressure drop could be decreased, since it significantly reduced the pressure loss during gas transportation.

Impact frequency variation of self-excited oscillation pulsed supercritical carbon dioxide jets

In order to obtain the impact frequency of resonant coal breaking by self-excited oscillation pulsed supercritical carbon dioxide(SC-CO_(2))jet,large eddy simulation was used to analyze the formation and development process of self-excited oscillation pulsed SC-CO_(2)jet,the variation of jet impact frequency in the nozzle and the free flow field,and the variation of jet impact frequency at different positions in the jet axis and under different cavity lengths.The test device of jet impact frequency was developed,and experiments were performed to verify the conclusions of the numerical simulations.The results show that the frequency of the self-excited oscillation pulsed SC-CO_(2)jet is different in the nozzle and the free flow field.In the nozzle,the frequency generated by the fluid disturbance is the same,and the jet frequency at the exit of the nozzle is consistent with that inside the nozzle.In the free flow field,due to the compressibility of CO_(2),the pressure,velocity and other parameters of SC-CO_(2)jets have obvious fluctuation patterns.This feature causes the impact frequency of the self-excited oscillation pulsed SC-CO_(2)jet to decrease gradually in the axis.Changing the cavity length allows the adjustment of the jet impact frequency in the free flow field by affecting the disturbance frequency of the self-excited oscillation pulsed SC-CO_(2)jet inside the nozzle.

Mechanism of the self-excited oscillation pipeline flocculator

This paper presents the research on the dynamic mechanism of flocculation based on the characteristcs of turbulent flow. The shearing force and the centrifugal force transferred by the vortex are the main forces to cause collision of flocculated grains in water and the shearing force is the primary one. Based on this mechanism, a new type of self-excited oscillation pipeline flocculator is designed.

Side force control on slender body by self-excited oscillation flag

Strong asymmetrical vortices appear on the leeward of slender body at high angles of attack, which has very unfavorable effect on the stability and control of the aircraft. A method is developed to control the side force of slender body at high angles of attack, and is verified in wind tunnel. A thin-film triangular self-excited oscillation flag is fixed at the tip of the slender body model whose semi-apex angle is 10°. Side force is approximately linearly proportional to roll-setting angle of self-excited oscillation flag at high angles of attack, and the slop of fitting straight line obtained by the least square method is -0.158. The linear relationship between side force and roU-setting angle provides convenience for developing side force control law of slender body at high angles of attack. Experimental data shows that the side force coefficients vary linearly with roll-setting angles when a specific plastic self-excited oscillation flag is used as the control flag. The range of side force coefficient and roll-setting angle are, respectively, -3.2 to 3.0 and -20° to 20°. The device is simple, effective, and is of great potential in engineering application.

Teleconnected Influence of the Boreal Winter Antarctic Oscillation on the Somali Jet： Bridging Role of Sea Surface Temperature in Southern High and Middle Latitudes

The teleconnection impact of the boreal winter Antarctic Oscillation（AAO） on the Somali Jet（SMJ） intensity in the following spring and summer is examined in this paper.The variability of the boreal winter AAO is positively related to the SMJ intensity in both spring and summer.The analyses show that the SST in southern high and middle latitudes seems to serve as a bridge linking these two systems.When the AAO is in strong positive phase,SST over the Southern Ocean cools in the high latitudes and warms in the middle latitudes,which persists into summer;however,the variability of SST in southern high and middle latitudes is also closely correlated to SMJ intensity.A possible mechanism that links SST variability with the AAO-SMJ relationship is also discussed.The AAO in boreal winter produces an SST anomaly pattern in southern high and middle latitudes through the air-sea coupling.This AAOrelated SST anomaly pattern modulates the local Ferrel cell anomaly in summer,followed by the regional Hadley cell anomaly in tropics.The anomalous vertical motion in tropics then changes the land-sea thermal contrast between the tropical Indian Ocean and the Asian continent through the variability of low cloud cover and downward surface longwave radiation flux.Finally,the land-sea thermal contrast anomaly between the tropical Indian Ocean and the Asian continent changes the SMJ intensity.The results from Community Atmosphere Model experiments forced by the SST anomaly in southern high and middle latitudes also confirm this diagnostic physical process to some extent.

Characteristics of Oscillation in Cavity of Helmholtz Nozzle Generating Self‑excited Pulsed Waterjet

Cavity flow oscillations in the axisymmetric cavity are critical to the operating efficiency of self-excited pulsed waterjets,which are widely employed in many practical applications.In this study,the behaviors of a turbulent flow in axisymmetric cavities causing cavity flow oscillations are investigated based on wall pressure characteristics.Experiments are performed using four Helmholtz nozzles with varying length-to-radius ratios at flow velocities of 20–80 m/s.Three orders of hydrodynamic modes in axisymmetric cavity are obtained through the spectral analysis of wall pressure.Based on the experimental results,the empirical coefficient of Rossiter’s formula is modified,and the values of the parameter phase lag and the ratio of convection velocity to free stream velocity are obtained as 0.061 and 0.511,respectively.In addition,the spectral peak with a relatively constant frequency shows that the flow-acoustic resonance is excited significantly.A modified model is introduced based on the fluidic networks to predict the lockon frequency.The results obtained can provide a basis for the structural optimization of the nozzle to improve the performance of self-excited pulsed waterjets.

OSCILLATION PHENOMENA IN FAR FIELD REGION OF PLANE JETS AT LOW REYNOLDS NUMBERS

Oscillation phenomena in far field region of plane jets are studied by lattice Boltzmann method over a range of Reynolds numbers （Re） from 16 to 65. Numerical results show that the instantaneous centerline velocities show periodic oscillation behavior in far field region when Re〉38. In contrast, the periodic behavior is invisible in corresponding flow field when Re≤38. For the cases of Re≤38, the exchange of momentum due to straining mo- tion gradually dominates the downstream flow filed, which qualitatively suggests the possibility of iet instability.

Shock oscillation in highly underexpanded jets

The oscillatory motions of shocks in highly underexpanded jets with nozzle pressure ratios of 5.60, 7.47, 9.34, and11.21 are quantitatively studied by using large eddy simulation. Two types of shock oscillations are observed： one is the Mach disk oscillation in the streamwise direction and the other is the shock oscillation in the radial direction. It is found that the Mach disk moves quickly in the middle of the oscillatory region but slowly at the top or bottom boundaries. The oscillation cycles of Mach disk are the same for different cases, and are all dominated by an axisymmetric mode of 5.298 k Hz. For the oscillation in the radial direction, the shocks oscillate more toward the jet centerline but less in the jet shear layer, and the oscillation magnitude is an increasing function of screech amplitude. The cycles of the radial shock oscillation switch randomly between the two screech frequencies for the first two cases. However, the oscillation periodicity is more complex for the jets with high nozzle pressure ratios of 9.34 and 11.21 than for the jets with the low nozzle pressure ratios of 5.6 and 7.47. In addition, the shock oscillation characteristics are also captured by coarse mesh and Smagorinsky model,but the coarse mesh tends to predict a slower and weaker shock oscillation.

Complex dynamics of an archetypal self-excited SD oscillator driven by moving belt friction

We propose an archetypal self-excited system driven by moving belt friction, which is constructed with the smooth and discontinuous （SD） oscillator proposed by the Cao et al. and the classical moving belt. The moving belt friction is modeled as the Coulomb friction to formulate the mathematical model of the proposed self-excited SD oscillator. The equilibrium states of the unperturbed system are obtained to show the complex equilibrium bifurcations. Phase portraits are depicted to present the hyperbolic structure transition, the multiple stick regions, and the friction-induced asymmetry phenomena. The numerical simulations are carried out to demonstrate the friction-induced vibration of multiple stick-slip phenomena and the stick-slip chaos in the perturbed self-excited system. The results presented here provide an opportunity for us to get insight into the mechanism of the complex friction-induced nonlinear dynamics in mechanical engineering and geography.

Numerical Analysis on the Oscillator of External Stimulation and Self-Excited

In this article, the performance of the novel external stimulation oscillator and the optimized self- excited oscillator has been investigated by CFD numerical simulation. The efficiency and stimulation mechanism of the two kinds of oscillators were analyzed in numeric form. The cycle proportion of wall-attached jet switching time was calculated separately at different operating frequency. Additionally the reason for the difference of two oscillators was analyzed and explained.

Satellite Measurements of the Madden–Julian Oscillation in Wintertime Stratospheric Ozone over the Tibetan Plateau and East Asia

We investigate the Madden-Julian Oscillation （MJO） signal in wintertime stratospheric ozone over the Tibetan Plateau and East Asia using the harmonized dataset of satellite ozone profiles. Two different MJO indices -- the all-season Real-Time multivariate MJO index （RMM） and outgoing longwave radiation-based MJO index （OMI） -- are used to compare the MJO- related ozone anomalies. The results show that there are pronounced eastward-propagating MJO-related stratospheric ozone anomalies （mainly within 20-200 hPa） over the subtropics, The negative stratospheric ozone anomalies are over the Tibetan Plateau and East Asia in MJO phases 4-7, when MJO-related tropical deep convective anomalies move from the equatorial Indian Ocean towards the western Pacific Ocean. Compared with the results based on RMM, the MJO-related stratospheric column ozone anomalies based on OM1 are stronger and one phase ahead. Further analysis suggests that different sampling errors, observation principles and retrieval algorithms may be responsible for the discrepancies among different satellite measurements. The MJO-related stratospheric ozone anomalies can be attributed to the MJO-related circulation anomalies, i.e., the uplifted tropopanse and the northward shifted westerly jet in the upper troposphere. Compared to the result based on RMM, the upper tropospheric westerly jet may play a less important role in generating the stratospheric column ozone anomalies based on OMI. Our study indicates that the circulation-based MJO index （RMM） can better characterize the MJO- related anomalies in tropopause pressure and thus the MJO influence on atmospheric trace gases in the upper troposphere and lower stratosphere, especially over subtropical East Asia.

HARMONIC, SUBHARMONIC, SUPERHARMONIC, SIMULTANEOUS SUB/SUPER HARMONIC AND COMBINATION RESONANCES OF SELF-EXCITED TWO COUPLED SECOND ORDER SYSTEMS TO MULTI-FREQUENCY EXCITATION

Harmonic, subharmonic, superharmonic, simultaneous sub/super harmonic, and combination resonances of the additive type of self-excited two coupled-second order systems to multi-frequency excitation are investigated. The theoretical results are obtained by the multiple-scales method. The steady state amplitudes for each resonance are plotted, showing the influence of the different parameters. Analysis for each figure is given. Approximate solution corresponding to each type of resonance is determined. Stability analyses are carried out for each case.

Experiments on Deflecting & Oscillating Waterjet

A new type jet, the oscillating & deflecting jet, is put forward and its oscillating and deflecting characteristics are investigated. The nozzle of the self-oscillating & deflecting water jet consists of an upstream nozzle, a downstream nozzle, an oscillating chamber and two switches. It is experimentally shown that the deflective angle may reach 9.53 degree. The generated pressure fluctuation is very regular and the jet can efficiently increase the ability for breaking and cutting by eliminating the water cushion effect associated with a continuous jet.

Effect of Oscillating Jet Velocity on the Jet Impingement Cooling of an Isothermal Surface

Numerical investigation of the unsteady two-dimensional slot jet impingement cooling of a horizontal heat source is carried out in the present article. The jet velocity is assumed to be in the laminar flow regime and it has a periodic variation with the flow time. The solution is started with zero initial velocity components and constant initial temperature, which is same as the jet temperature. After few periods of oscillation the flow and heat transfer process become periodic. The performance of the jet impingement cooling is evaluated by calculation of friction coefficient and Nusselt number. Parametric study is carried out and the results are presented to show the effects of the periodic jet velocity on the heat and fluid flow. The results indicate that the average Nusselt number and the average friction coefficient are oscillating following the jet velocity oscillation with a small phase shift at small periods. The simulation results show that the combination of Re =200 with the period of the jet velocity between 1.5 sec and 2.0 sec and high amplitude (0.25 m/s to 0.3 m/s) gives average friction coefficient and Nusselt number higher than the respective steady-state values.

Interannual Variation of the Onset of Yunnan’s Rainy Season and Its Relationships with the Arctic Oscillation of the Preceding Winter

Based on an analysis of the circulation in May associated with the interannual variation of the onset of Yunnan’s rainy season, this study examined the rela-tionship between Arctic Oscillation (AO) and the onset timing of the rainy sea-son by using the NCEP/NCAR reanalysis and observational precipitation data for 1961-2010. The results indicated that, on an interannual time scale, intense Asian summer monsoon and an active EU-pattern wave train circulation in its positive phase, associated with a cold cyclonic cell covering the western part of the East Asian subtropical westerly jet (EASWJ), jointly contributed to the onset of the rainy season in May. Otherwise, the onset might be suppressed. The cold cyclonic cell over East Asia likely led to the southward shift and enhancement of EASWJ as well as its secondary circulation around the jet entrance, which could provide a favorable dynamic and thermal condition for rainfalls in Yunnan as was revealed in previous studies on 10 - 30-day time scale. Further examination showed that the preceding wintertime AO played a significant role in the timing of the onset of the rainy season before the mid-1980s’ by mostly modulating the wave-train-like circulation over East Asia in May. During that time period, when the AO index of the previous winter was positive (negative), Yunnan’s rainy season tended to begin earlier (later) than normal. Correspond-ingly, the precipitation in May was also closely linked to wintertime AO.

Characteristics and Formation Mechanisms of Low-Level Jets in Northeastern China

This study examines low-level jets(LLJs)across Northeastern China during both warm(June-September)and cold seasons(December-March)from 1957 to 2021,using fifth generation of the European Centre for Medium-Range Weather Forecasts reanalysis data with 25-km resolution.LLJs manifest in two prominent regions,one along the leeward flank of the Da Hinggan Ling Mountains in the cold season and another at the center of Northeastern China in the warm season.The intricate interplay between ambient circulation and terrain shapes LLJ distribution,altitudes,wind directions,diurnal cycles,and seasonal diversities.During the warm season,prevailing southwesterly LLJs are found at 925 hPa,while the cold season features stronger and more frequent northwesterly LLJs at 875 hPa.Analysis of the diurnal patterns reveals distinctive behaviors of LLJs in the cold and warm seasons.During the warm season,the single peak in LLJ occurrence emerges around midnight;conversely,in the cold season,LLJs are most frequent shortly before midnight,with an additional sub-peak in the morning.A momentum budget analysis establishes mechanisms underlying these two distinct diurnal variations.In both seasons,the diurnal variation of LLJs is predominately driven by an inertial oscillation and mountain-valley circulations.However,the sub-peak observed in the cold-season morning arises from the thermodynamic and dynamic interaction between the low-level atmosphere and complex terrain.

Internal Vibration and Synchronization of Four Coupled Self-Excited Elastic Beams

The vibration behavior and the synchronization between some internal points of four coupled self-excited beams are numerically studied. Coupling through the root of the beams is considered. The transverse displacements of the internal points and the beam tips are monitored, and the power spectra of the resulting time series are employed to determine the oscillation frequencies. The synchronization between beams is analyzed using phase portraits and correlation coefficients. Numerical results show multiple frequencies in the vibration pattern, and complex patterns of synchronization between pairs of beams.

基于脉冲水射流喷嘴结构优化的数值模拟

利用Fluent软件对弧形式碰荡壁自激脉冲喷嘴进行数值模拟,以涡环大小和脉冲射流峰值速度作为判断喷嘴产生脉冲效果的依据,通过分析振荡腔长度、碰撞壁弧度和振荡腔直径对水射流的影响,设计了最优弧形式振荡壁自激脉冲喷嘴。研究表明,弧形式振荡壁自激脉冲喷嘴的最优几何参数为:振荡腔长度为4.5 mm,碰撞壁弧度半径为9.5 mm,振荡腔直径为10 mm。